Since carbon fiber has a high specific strength and specific modulus compared to other fibers, as a reinforcing fiber for composite materials, in addition to conventional sporting good applications or aerospace applications, it has also been widely applied to general industrial uses such as for automobile, civil engineering and architecture, compressed container and wind turbine blade, and further improvement in productivity and production stability are highly demanded.
Among the carbon fibers, polyacrylonitrile (hereafter, may be abbreviated as PAN)-based carbon fiber, which is most widely used, is industrially produced by subjecting a spinning solution composed of a PAN-based polymer, which is precursor of the fiber, to a wet spinning, a dry spinning or a dry-wet spinning to obtain a precursor fiber of carbon fiber, and then it is converted to a stabilized fiber by heating at a temperature of 200 to 400° C. under an oxidizing atmosphere and carbonized at a temperature of at least 1,000° C. under an inert atmosphere.
The improvement of productivity of the PAN-based carbon fiber has been tried in any view point of spinning, stabilization and carbonization of the precursor fiber of carbon fiber. Among them, the improvement of productivity of the PAN-based precursor fiber of carbon fiber was difficult from the following problems. That is, in the spinning for obtaining the PAN-based precursor fiber of carbon fiber, its productivity is limited by a critical draft ratio at spinning accompanied by characteristics of the PAN-based polymer solution and a critical draw ratio accompanied by its coagulation structure, and when spinning speed is increased to improve productivity, drawability decreases to cause an unstable production, and when spinning speed is decreased, although production is stabilized, productivity decreases, and accordingly, there was a problem that an improvement of productivity and a stabilization were hard to be compatible.
There is spinning method as a factor which greatly affects the critical draft ratio at spinning, and accordingly, the critical draft ratio is explained in view of the spinning method. The dry spinning method is a method in which a spinning solution is extruded from spinneret holes in a high temperature gas atmosphere and the solvent is evaporated to concentrate and solidify, and since its taking up speed is bound by the evaporation speed of the solvent, there are defects such as that a very long spinning cylinder becomes necessary accompanied by an increase of the taking up speed.
On the other hand, the wet spinning method is a method in which a spinning solution is extruded from spinneret holes in a coagulation bath, but since the coagulation is started from just after the spinning solution is extruded from the spinneret holds, although a substantial draft ratio at spinning increases depending on increasing of the taking up speed, there is a problem that a fiber breakage occurs at spinneret surface, and accordingly, there is a limit to make the taking up speed high.
Furthermore, in the dry-wet spinning method, since a spinning solution is once extruded in the air (air gap) and then introduced into a coagulation bath, and since the substantial draft ratio at spinning is absorbed in the original liquid flow in the air gap to enable a high speed spinning, several proposals have been made so far. For example, a technique of improving taking up speed by reducing resistance of the coagulation bath by using a flow-down type coagulation bath (JP-S59-21709 A)) is proposed. However, in this proposal, although taking up speed can be greatly improved, (1) since a spinneret of a specified shape is used, a precursor fiber of a fine single fiber thickness cannot be obtained, (2) the structure of coagulation bath is complicated and it not a technique capable of realizing industrially, and (3) because of a relation between a slit of flow-down cylinder outlet and a thickness of fiber bundle to be passed, there was a problem such as that its operation or production stability is impaired.
Furthermore, a technique is proposed in which, by controlling polymer concentration of the spinning solution, spinning solution viscosity is decreased to improve handling at filtering operation, to improve the draft ratio at spinning (JP-S64-77618 A). However, according to this proposal, although an improvement effect is found as that the draft ratio at spinning is 10, (1) it is not economical since the polymer concentration is low and an amount of solvent used is large, and (2) there are problems that coagulation speed in the coagulation bath decreases and voids are generated inside and a dense structure cannot be obtained.
In general, it is known that an increase of viscosity during a large extensional deformation in a melt-molding such as melt-spinning is advantageous to prevent an unstable flow. As one technique for that, a method of adding a small amount of an ultra-high molecular weight polymer is mentioned (Journal of the Society of Rheology, Japan, P215, No. 25 (1997)). In the case where such a polymer is used as a polymer for melt-spinning, it is known that spinnability is improved. However, application of this technique to a solution spinning which is an ordinary spinning method for PAN-based polymer has almost not been tried.
Mixing 2 kinds of polymer of PAN-based polymer different in molecular weight distribution means to make molecular weight distribution wide (broad). As method for controlling the molecular weight distribution, several proposals have been made so far. For example, a method is proposed to obtain a high strength and high modulus PAN-based fiber by using a polymer of which molecular weight distribution is narrowed as a weight average molecular weight (hereafter, may be abbreviated as Mw) is 400,000 or more and a molecular weight distribution (Mw/Mn) which is the ratio of Mw to number average molecular weight (hereafter, may be abbreviated as Mn) is 7.0 or less (JP-S61-97415 A). As represented by this proposal, conventionally, it has been proposed that a narrowing molecular weight distribution is preferable for the precursor fiber of carbon fiber.
In particular, in aircraft applications, a carbon fiber of which compressive strength and tensile modulus are compatible in a high level is demanded. In carbon fibers, as the highest temperature in carbonization step becomes higher, the tensile modulus of the obtained carbon fiber can be made higher, but accompanied by a growth of carbon net planes, compressive strength of the obtained carbon fiber decreases. That is, the relation between tensile modulus and compressive strength of carbon fiber is in a trade-off relation. To make the compressive strength and tensile modulus in this trade-off relation compatible, other than controlling the carbonization temperature, regarding techniques for increasing the compressive strength and tensile-modulus, several proposals have been made so far.
As techniques for improving the compressive strength of carbon fiber, for example, techniques are proposed in which carbon fiber is subjected to an ion implantation to decrystallize graphite crystal, or cross-sectional shape of precursor fiber to be used is made into non-circular, to increase geometrical moment of inertia (JP-H3-180514 A and JP-H3-185121 A). However, in the former proposal, carbon fiber can be treated only by little by little at a high vacuum, and in the latter proposal, it is difficult to maintain a stable cross-sectional shape and there is a problem in uniformity of final product, and both proposals are difficult to be applied industrially.
It is known that, to improve tensile modulus of carbon fiber, an increase of degree of orientation of the carbon fiber to be obtained by drawing the fiber at stabilization-carbonization is effective. However, only by the increase of draw ratio, a generation of fuzz or fiber breakage is induced, and lowering of production stability or lowering of grade of the carbon fiber to be obtained is unavoidable. A technique of improving stabilization of drawing by controlling stabilization-carbonization conditions is also proposed (JP-2004-91961 A and JP-2004-197278 A). However, it cannot be said that the level of drawing is high, and the improvement effect of tensile modulus by the drawing is small.
It could therefore be helpful to provide a polyacrylonitrile-based polymer suitable for producing a precursor fiber of carbon fiber capable of increasing spinning speed, and in addition, capable of increasing draft ratio at spinning, and a production method thereof. It could be helpful, by using the polyacrylonitrile-based polymer solution, to provide a method for producing a high grade precursor fiber of carbon fiber with little fuzz, without spoiling productivity. It could further be helpful to provide a method capable of stably producing also in a stabilization-carbonization step, a high grade carbon fiber in which the above-mentioned high grade precursor fiber of carbon fiber is used. It could still further be helpful to provide a carbon fiber excellent in both of compressive strength and tensile modulus and a production method thereof without spoiling productivity and processability.